Thermal imaging systems provide a visual display of a scene in which the infrared light due primarily to emitted blackbody radiation from a scene is imaged, as opposed to visible imaging systems in which scattered or reflected visible light is imaged. By inserting one or more calibrated blackbodies within the thermal imaging systems, it is possible to correlate the grey shades of an image to the apparent temperature of objects within the image. Such a system is then called an imaging radiometer.
The typical fast framing thermal imaging system or FLIR (forward looking infra-red) collects, spectrally, filters and focuses the infrared radiation within the field of view onto an array of detectors. The detectors convert the optical signals into electrical signals which are amplified and processed for display on a video monitor. The image for a high performance system is typically provided on a T.V.-type monitor operating at T.V. frame rates. This is all accomplished in real time.
The major applications to date for thermal imaging have been for military and intelligence purposes, and the bulk of the prior of which we are aware is directed to accomplishing military purposes. For example, thermal imagers have been used in rifle scopes and gun sights and for surveillence and airborne weapon delivery. The military requirements are such that the typical thermal imager used for military purpoes is much too costly for commercial applications and has other characteristics which make it unsuited for direct application to many commercial fields.
All of the high performance thermal imaging systems use one of two mechanical raster scan concepts in combination with an array of cryogenically cooled detectors. One of these concepts employs a large array of up to 180 detectors oriented perpendicular to line scan dimension. The signals from the detectors are amplified and directly displayed by synchronously scanned light-emitting diodes or multiplexed to drive a cathode ray tube. This concept is illustrated in U.S. Pat. No. 3,760,181 issued to Daly et al.
A second approach is disclosed in Laakmann U.S. Pat. No. 3,723,642. In this implementation, a short array of detectors, such as ten to thirty detectors, is scanned two-dimensionally across the image. The detectors are oriented parallel to the line scan dimension of the T.V. raster to be generated. The signals from the detectors are summed appropriately in a delay line and processed to provide the image. Since each detector sees a perfect cold stop, this implementation provides thermal sensitivity equal to the less efficient Daly et al implementation.
An optical scanner which can accomplish two-dimensional scanning at commercial T.V. rates is described in Wheeler U.S. Pat. No. 3,764,192. This scanner is limited to scanning relatively small normalized apertures typically required for military applications. The scanner in military applications is typically used with ultra-high resolution telescopes, and accordingly, it is necessary to minimize the size of the scanner entrance pupil and/or the field angle unless the aberrations are allowed to become excessive. Consequently, the scanner is configured for small D.theta. products of the order of 0.8 to 1 where D is the effective entrance (i.e., where the radiation enters the scanner) diameter in inches of the collimated beam and .theta. is the detector aximuth angular subtense in milliradians. At the entrance aperture, the effective diameter is the diameter of a circle which has an area equal to the cross-sectional area of the collimated beam.
The small D.theta. products require the use of a large number of detectors in order to provide adequate sensitivity. The large number of detectors very substantially increases the cost of the unit to such an extent that a thermal imaging system using a large number of detectors is not feasible for many commercial applications.